Engine control systems may use spark retard from MBT, or borderline spark settings, to provide fast response torque reduction. For example, spark retard may be used during transmission shifts to provide a smoother shift feel. Following the transmission shift, initial spark settings may be resumed. However, the retarded spark setting can lead to late combustion cycles wherein combustion occurs at a time or crank angle location later than intended. The late burn can potentially lead to abnormal combustion events such as misfires or pre-ignition events. Boosted engines may be more susceptible due to their higher sensitivity to ignition output requirements.
Accordingly, strategies have been developed for reducing pre-ignition onset by late combustion events. One example approach is shown by Wozniak et al. in U.S. Pat. No. 6,883,497. Therein, an engine controller determines if a cylinder misfire event has occurred in a cylinder due to spark retard in the cylinder. If no misfire occurs, a higher likelihood of pre-ignition is anticipated, and accordingly a mitigating operation is performed, such as cylinder fuel cut-off.
However, the inventors herein have identified a potential issue with such an approach. While the approach of Wozniak et al. may mitigate pre-ignition in the cylinder undergoing late combustion, the approach may be unable to address pre-ignition in other affected cylinders. Specifically, due to randomness of combustion preparation, there is a risk that the late combustion event in the given cylinder may be even later than intended. The later than intended initiation of combustion may raise the likelihood of pre-ignition in the given cylinder as well as in one or more neighboring cylinders. For example, the late combustion event in the given cylinder may introduce high amount of hot exhaust residuals into one or more adjacent cylinders by forcing open the exhaust valve of the adjacent cylinder(s). The excess hot residual received in the adjacent cylinders can increase their propensity for pre-ignition. In addition, the excess hot residual can increase the amount of fresh cylinder charge inducted (via turbine spinning), leading to a further increase in the propensity for pre-ignition. Consequently, engine degradation may occur.
Thus in one example, some of the above issues may be addressed by an engine method comprising, in response to timing of combustion in a cylinder being later than a threshold, selectively increasing spark ignition output for the combustion. In this way, unintended combustion delays may be reduced.
For example, during a transmission shift, transient torque control may be enabled by retarding spark timing. If spark timing in a cylinder is retarded more than a threshold amount, there may be a risk that the cylinder combustion timing will be even later than intended. Thus, in response to the spark retard applied being more than the threshold amount, a dwell time of the ignition coil of the cylinder may be extended so as to temporarily increase the ignition output. The dwell time may be increased as a distance of the retarded spark timing from a threshold timing increases. For example, the dwell time may be increased with a factor that is based on the torque reduction ratio achieved via the spark retard. The increased ignition output may be applied for the given cylinder combustion event so as to better enable the combustion event to start at a desired timing, and not be further delayed. Then, during a subsequent combustion event, where spark timing with less retard is resumed, the ignition dwell time may be lowered.
In this way, by temporarily increasing spark ignition output in a cylinder operating with spark retard, combustion timing delays incurred due to randomness of combustion preparation can be reduced. By improving the likelihood that a cylinder combustion event will occur at a desired timing, and not any later, pre-ignition in the given cylinder, as well as in a cylinder configured to receive exhaust residuals from late combustion in the given cylinder, may be reduced. Overall, engine degradation due to pre-ignition can be mitigated.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.